The field of the present invention is electronic devices in the form of analog to digital converters. More particularly, the described analog to digital converter is configured for operation in support of a wireless communication device.
Wireless communication devices are widely used to communicate data, voice, and other information between physical locations without the use of wires. With the expanded use of wireless communication devices, a need has grown for faster and more reliable communication devices. At the same time, users are demanding that wireless communication devices be more compact and be easier to transport. For example, it is now common for a handheld mobile telephone handset to not only provide for voice communication, but these same handsets more frequently enable access to information by providing remote access to data networks such as the Internet.
Typically, a wireless communication device has a wireless receiver for receiving an analog signal transmitted from a transmitter. Often the analog signal is a radio frequency signal having voice or data modulated on to a carrier. The wireless receiver accepts and amplifies the analog signal, and may provide selected filtering. For example, band pass filters may be applied to the analog input signal to block the further processing of signals at adjacent frequencies.
Once the analog signal has been amplified and processed, the wireless communication device converts the analog signal into lower frequency digital data for further processing within the wireless communication device. The conversion of the analog signal to a digital representation generates a signal that is easier to process, for example, because it is operating at a lower frequency. This process is generally referred to as analog to digital conversion.
Although various technologies may be used to implement an analog to digital conversion, a commonly used analog to digital converter relies on a delta-sigma converter to convert an analog signal into a digital bit stream. The digital bit stream may then be further processed and filtered to provide a digital representation of the information carried in the analog input signal.
Analog to digital converters are designed to operate within an expected range of input level. While receiving input levels in the proper operating range, the digital output of the analog to digital converter provides a representation of the information carried by the analog signal. However, if the input level to the analog to digital converter exceeds the expected operating level, then the converter may be placed into a state of saturation. When saturated, the digital data output from the converter reflects the saturation condition, not the information carried in the analog signal. For example, once the converter is saturated, the digital data output from the converter may become all 1""s, or may start oscillating and generate random data. Accordingly, any information from the input signal is lost. Since the converter is no longer providing accurate or useful digital information, any further processing of the digital data is futile. For example, if the analog signal carries informative information to a wireless handset, and the converter in the handset receiver saturates, the received informative signal will drop out, and the call may be lost.
In wireless receivers, the dynamic range at the input of the analog to digital converter is determined by the amount of filtering provided on the signal input to the converter as well as the fading characteristics of the received signal. For example, signals at adjacent frequencies may be sufficiently strong to drive the analog to digital converter into a state of saturation.
Accordingly, in known, conventional wireless devices, the input signal to the analog to digital converter is attenuated to a level that allows the analog to digital converter to operate at the maximum input levels expected to be received. Since such conditions only rarely occur, the wireless device is nearly always operating with an unnecessarily attenuated input signal. Such attenuation not only reduces the sensitivity of the receiver, limiting the device""s useful operating range, but also increases the cost of manufacturing the wireless device as more sensitive components are required. However, even with such an attenuated input signal, it is likely that the wireless device will occasionally receive input signals exceeding the expected maximum level. In such a case, the analog to digital converter will saturate and any active information transfer will fail.
Therefore, there exists a need to accommodate unusually large input signals by avoiding saturating the analog to digital converter, while maintaining adequate sensitivity levels.
It is therefore an object of the present invention to provide a saturation compensating analog to digital converter. It is a further separate objective of the present invention to provide a saturation compensating analog to digital converter without modifying the base band portion of a wireless communication device. Therefore, to overcome the deficiencies of conventional systems, and to meet the stated objectives, a saturation compensating analog to digital converter is provided.
Briefly, the saturation compensating analog to digital converter has converter circuitry receiving an analog signal and outputting converted data. The converted data from the converter circuitry is processed and filtered to provide a digital data output. The digital data output is received into state machine before being transmitted for later-stage processing. When the converter circuitry is operating close to a saturated condition, a saturation detector generates a saturation signal. The saturation signal is received at a variable gain circuit which adjusts the gain of the input signal to the converter circuitry. The state machine also receives the saturation signal and provides an upshift of the digital data to compensate for the associated reduction in input gain provided by the variable gain circuit. In operation, the variable gain circuit is initially set to its maximum output, thus providing the maximum possible input signal to the converter. If a near-saturation conditional is detected, the variable gain circuit is stepped down, and the state machine provides an associated one bit step up. Such saturation compensation is continued to enable the converter circuitry to operate without saturating.
Advantageously, the saturation compensating analog to digital converter can be implemented without requiring any change to the base band receiver. For example, if the input signal to the converter is attenuated to avoid a saturation condition, the resulting output digital data is correspondingly upshifted to compensate for the reduced input gain. The detection and compensation for a saturation condition, is thereby accommodated in a manner that can be easily and efficiently integrated into existing wireless communication devices.
These and other features and advantages of the present invention will be appreciated from review of the following detailed description of the invention, along with the accompanying figures in which like reference numerals refer to like parts throughout.